If Gustav heads into southeast Louisiana, scientists and engineers agree that large swaths of the region could be at great risk of flooding from even a moderate storm surge, especially neighborhoods near the Industrial Canal and on the West Bank of Jefferson Parish.

More than $2 billion in repairs and improvements made to the system since Katrina cracked it open three years ago have strengthened some weak spots.

In particular, floodgates on three New Orleans outfall canals -- two of which broke through their floodwalls causing catastrophic flooding during Katrina -- should protect neighborhoods from surges flowing through Lake Pontchartrain. And new levees are giving protection to the Company Canal and Harvey Canal north of Lapalco Boulevard.

But almost $13 billion in work remains to be done before the region is protected from a 100-year storm -- about the size of Hurricane Rita -- and that means much of the hurricane protection system remains at risk.

In many cases, there's nothing that can be done to beef up inadequate flood defenses if Gustav strikes early next week. East of the Mississippi River, for example, the system's Achilles heel remains the Industrial Canal area, where $695 million worth of structures are planned at the confluence of the Gulf Intracoastal Waterway and the Mississippi River-Gulf Outlet. But that work, still being designed, won't start to provide any storm surge protection until this time next year.

Despite some higher Intracoastal Canal levees and new armoring against erosion, communities all around the Industrial Canal remain exposed to the potential for major flooding.

'A close eye' on stretch

Corps contractors were hustling Wednesday to fill and place hundreds of massive sand baskets along a particularly risky 1,800 feet of floodwall on the Industrial Canal's west side between Interstate 10 and the lake.

Engineers are banking on those Hesco baskets, weighing 3,000 pounds each, to keep surge off that troubled stretch of wall.

The extra protection is deemed so critical that National Guard troops are on standby to help finish the work if necessary, corps decision-makers said.

Levee officials said they'll also pay close attention to the 1,800 feet of floodwall on the canal's opposite bank after computer calculations indicated seepage problems.

"They show theoretical seepage, but there was no physical evidence of seepage there after Katrina, and there's no evidence of seepage now, " said engineer Bob Turner, executive director of the Southeast Louisiana Flood Protection Authority-East.

"But we'll be keeping a close eye on that area, and if there's any evidence of seepage, we'll get experts out to look at it right away and determine what to do at that point, " he said.

Hundreds of Hesco sand baskets are already in place in East Jefferson and other parts of the system to bolster the height of low stretches of levee. If Gustav approaches, heavy equipment is on standby to close gaps that provide levee access to residents and businesses.

West Bank mobilizes

On the West Bank, crews will close gaps in a temporary line of defense east of the Harvey Canal in advance of Gustav.

West Bank levee officials also plan to use the mammoth sand baskets to close a 500-foot gap in an earthen levee north of the Intracoastal Canal between Belle Chasse Highway and the Harvey Canal.

"What we're looking at is expectation of a hit, " said David Bindenwald, outgoing president of the West Bank levee board. "We're gearing up with that in mind."

If Gustav is still a threat to the region today, Turner said he would meet with executive directors of the Orleans, East Jefferson and Lake Borgne Basin levee districts to evaluate any gaps in the system.

On the Orleans lakefront for example, Canal Boulevard and six other roadways now dip three to four feet below the recently raised levees on both sides of them.

As part of building a 100-year system, the corps ultimately will close those gaps by either elevating the streets or adding floodgates.

"Those areas weren't overtopped during Katrina, and the elevations are close to 100-year (protection levels) now, but that will be part of what we look at, " Turner said.

Turner and the directors also must determine the sequences in which floodgates and valves will be closed in the three districts.

There are 200 gates and 60 valves in the Orleans district alone that must be closed in a sequence that ensures getting the job done without disrupting evacuation routes or businesses.

Turner also wants the public to know that the districts routinely sandbag floodgates to reduce seepage as water levels rise.

"I don't want people to be alarmed if they see us doing this, " he said. "Gates aren't watertight."

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Meghan Gordon contributed to this report.Sheila Grissett can be reached at sgrissett@timespicayune.com or 504.717.7700.